Abstract: A riser reactor which includes a wall extending downward to an enlarged housing at the bottom of the riser reactor, where the housing has (a) an inlet for introducing catalyst particles into the housing and includes a first fluidization device, where the length of the inlet is from two to three times the diameter of the inlet, and (b) a second fluidization device below the first fluidization device, and (c) where the housing has above the inlet a centrally arranged diamond-shaped directing structure secured to the wall, where the directing structure has a lower and upper portion and where the directing structure and the wall define a passage, where the lower portion of the directing structure and the wall defines an upwardly converging portion in the passage and where the upper portion of the directing structure and the wall define an upwardly diverging portion in the passage.